1. Field of the Invention
This invention relates generally to data access arrangement devices, which facilitate data transfer between high speed analog modem devices and a telephone central office line, and more particularly to data access arrangement devices employing solid state isolation techniques without the use of transformers or linear opto-couplers.
2. Description of the Related Art
Data Access Arrangement (DAA) devices connect a central office telephone line with end-user equipment. The end-user equipment is typically a high speed analog modem device (e.g., V.32bis or V.34 modem), but DAA devices may also be used with facsimile machines, answering machines, key telephones, two-wire transceivers for short haul modems, and other types of user-defined equipment. The DAA device typically provides line monitoring, filtering, isolation, protection, and/or signal conversion. Conventional DAA devices use transformers to provide some of these functions, particularly isolation. Also, the prior art has used solid state DAA's that use signal modulation to pass through isolations.
The advent of high speed modems and the Personal Computer Memory Card International Association (PCMCIA) card standard, however, has resulted in more stringent demands on systems employing DAA devices, and hence, on DAA devices themselves. For example, high speed modems require that DAA devices function at higher bit rates while simultaneously meeting stringent noise and distortion requirements. The PCMCIA card standard introduces size and weight requirements.
Conventional transformer-based DAA devices have difficulty meeting the PCMCIA size and weight requirements and also have difficulty meeting the performance requirements at higher speeds. As a result, solid-state, transformerless DAA devices have been attempted. For example, opto-couplers have replaced transformers in some DAA device designs. These designs, however, are typically limited to low speed modems due to an inability to meet the stringent noise and distortion requirements at higher speeds; or else they require linear opto-couplers, which can be prohibitively expensive. In another approach, capacitors have replaced transformers as the isolation device. Capacitor designs, however, typically require redundant circuitry in the signal path, resulting in an overall reduction of the system signal-to-noise ratio.
Accordingly, there is a need for a transformerless DAA device which operates at high speeds while meeting stringent noise and distortion requirements but which is not prohibitively expensive.